The present invention generally relates to antennas.
There has been a theoretical limit on the gain-bandwidth product that is achievable by an antenna. This limit applies whether the antenna is electric (i.e., charge-coupled) or magnetic (i.e., flux-coupled) in nature. Usually, increasing bandwidth (or decreasing Q) leads to a decrease in gain over the bandwidth of interest. There continue to be new results reporting ever closer encroachments on this limit.
Two types of conventional antennas will now be described with reference to FIGS. 1-8.
FIG. 1 illustrates an electrical dipole 108 and the electric and magnetic fields associated therewith.
As shown in the figure, a z-axis 102, an x-axis 106 and a y-axis 104 create a right-hand coordinate system. For purposes of discussion, in this example, electrical dipole 108 is disposed along z-axis 102. Electrical dipole 108 has an electrical field, represented by sample lines 110, resulting from the disposition of positive charge +Q in the positive portion of z-axis 102 and negative charge −Q in the negative portion of z-axis 102. In accordance with the “right hand rule,” electrical dipole 108 has a concentric magnetic field, represented by sample line 112.
For purposes of discussion, consider the x-y plane where line 112 intersects lines 110. In this plane, constant magnetic field strengths form continuous circles and follow a right hand vector orientation rule. The electric fields for electric dipole 108 are spatially orthogonal to the magnetic fields and their lines of force begin and end on the ends of the electric monopole (charge coupled). The electric fields and magnetic fields may be represented as vectors pairs, samples of which are shown as electric field vector 114 and magnetic field vector 116, and electric field vector 118 and magnetic field vector 120. The vector cross product of an electric field vector and magnetic field vector describe power flow that is radially outward from electric dipole 108.
In many applications, an electric dipole may be used as an antenna, wherein the length of the electric dipole antenna may be equal to one half of the wavelength of the first harmonic of an electromagnetic wave that may be transmitted/received. In regards to Earth-bound antenna applications, e.g., a conventional radio station antenna, an electric dipole may be cut in half, to form an electric monopole, wherein the Earth approximates an infinite ground plane and ideal ground. An electric monopole antenna would provide field characteristics similar to an electric dipole associated with FIG. 1. In particular, if the electric monopole were to correspond to the axis of the antenna, the power radiating from the antenna would radiate outward such that the length of the electric monopole antenna may equal one fourth of the wavelength of the first harmonic of an electromagnetic wave that may be transmitted/received. The field characteristics associated with an electric dipole (and the electric monopole) should be compared to a magnetic dipole, as described with reference to FIG. 2.
FIG. 2 illustrates a magnetic dipole 208 and the electric and magnetic fields associated therewith.
As shown in the figure, a z-axis 202, an x-axis 206 and a y-axis 204 create a right-hand coordinate system. For purposes of discussion, in this example, magnetic dipole 208 is disposed along z-axis 202. Magnetic dipole 208 generates lines of electric field, represented by sample line 212, that encircle it in the x-y plane. Magnetic dipole 208 generates lines of magnetic field, represented by sample lines 210, that begin and end on surfaces having a net magnetic flux density. Again, the electric fields and magnetic fields may be represented as vectors pairs, samples of which are shown as electric field vector 214 and magnetic field vector 216, and electric field vector 218 and magnetic field vector 220.
The vector cross product of an electric field vector and magnetic field vector describe power flow that is radially outward from magnetic dipole 208. It should be noted that if the magnitude of M equals the magnitude of η0J, then E(MD)=−H(J) and H(MD)=E(J), where J is the electric current density in A/m2, M is the magnetic current density in V/m2, E is the electric field intensity in V/m and H is the magnetic field intensity in A/m. In other words, because the electric and magnetic field vector pairs have a similar relationship in an electric dipole antenna and a magnetic dipole antenna, the outward radiating power flow is similar.
An electric monopole (or dipole) and a magnetic dipole may be used to create an antenna. An example of an electric dipole antenna will now be described with reference to FIGS. 3-4.
FIG. 3 illustrates a conventional electric monopole antenna 302 using an electrical monopole to transmit a signal.
As shown in the figure, electric monopole antenna 302 is on a ground plane 304. A transmitter 306 is arranged to provide a current 308 to electric monopole antenna 302. Changes in current 308 generate transmission signals 310 from electric monopole antenna 302.
Consider the situation where current 308 is disposed within electric monopole antenna 302 such that charges resemble the electric dipole discussed above with reference to FIG. 1. In this manner, power will radiate outwardly from electric monopole antenna 302. As the current alternates, the radiating power will similarly alternate, providing transmission signals 310, which radiate outwardly. In this manner, electric monopole antenna 302 is an active device, transmitting a signal. Electric monopole antenna 302 may also perform as a passive device, receiving a signal.
FIG. 4 illustrates conventional electric monopole antenna 302 using an electrical monopole to receive a signal.
As shown in the figure, electric monopole antenna 302 is on a ground plane 304. A receiver 406 is arranged to receive a current 408 from electric monopole antenna 302. Received signals 410 generate changes in current 408, which are provided to receiver 406.
Signals 410 are electromagnetic waves. Electric monopole antenna 302 includes a conducting material. The interaction of signals 410 effect electrons within the conducting material of electric monopole antenna 302 to produce an overall charge therein. Consider the situation where such charges disposed within electric monopole antenna 302 resemble the electric dipole discussed above with reference to FIG. 1. As the electromagnetic fields change within signals 410, the magnitude and/or polarity of the charges within electric monopole antenna 302 similarly change. This change in the charge is current 408 (and similarly may be a change in current 408). Receiver 406 is able to receive current 408, and changes therein, to decode signals 410. In this manner, electric monopole antenna 302 is a passive device, receiving a signal. As mentioned above, a magnetic dipole may be additionally be used as an antenna.
An example of a magnetic dipole antenna will now be described with reference to FIGS. 5-7.
FIGS. 5A-C illustrate a conventional stacked magnetic tile core magnetic dipole antenna.
As shown in FIG. 5A, a stacked core 502 includes tile 504, stacked on tile 506, stacked on tile 508, stacked on tile 510. The material in each tile is used to increase magnetic field density.
As shown in FIG. 5B, an electrical excitation component 512 is disposed perpendicular to the length of stacked core 502.
As shown in FIG. 5C, a transmitter 514 is arranged to provide a current 516 to electrical excitation component 512 and then to ground 518. Current 516 generates concentric magnetic field lines, represented by sample dotted line 520, around electrical excitation component 512. The concentric magnetic field around electrical excitation component 512 induces magnetic fields within stacked core 502, wherein the magnetic fields within stacked core 502 exit one end of stacked core 502 and return into the other end of stacked core 502 so as to make a closed loop of field lines, an example of which is represented as represented by field line 522. Here the direction of propagation of the dynamic electromagnetic field associated with field line 522 is normal to the H vector and the E vector associated with field line 522, as represented by arrow 524.
In this example, field line 522 resembles the magnetic dipole discussed above with reference to FIG. 2. In this manner, power will radiate outwardly from stacked core 502.
FIG. 6 illustrates a conventional stacked magnetic tile core magnetic dipole antenna 602 using the magnetic dipole to transmit a signal.
As shown in the figure, conventional stacked magnetic the core magnetic dipole antenna 602 is disposed to receive a current 604 from a transmitter 514. Changes in current 604 generate transmission signals 606 from magnetic dipole antenna 602. In this example, antenna 602 includes stacked core 502 and electrical excitation component 512 of FIG. 5.
Consider the situation where current 604 is fed to magnetic dipole antenna 602 such that generated magnetic dipole fields within stacked core 502 resemble the magnetic dipole fields associated with the magnetic dipole discussed above with reference to FIG. 2. In this manner, power will radiate outwardly from magnetic dipole antenna 602. As the current alternates, the radiating power will similarly alternate, providing transmission signals 606, which radiate outwardly. In this manner, magnetic dipole antenna 602 is an active device, transmitting a signal. Magnetic dipole antenna 602 may also perform as a passive device, receiving a signal.
FIG. 7 illustrates conventional stacked magnetic tile core magnetic dipole antenna 602 using the magnetic dipole to receive a signal.
As shown in the figure, conventional stacked magnetic tile core magnetic dipole antenna 602 is arranged to receive signals 702. Changes in signals 702 generate changes in a current 704, which is provided to a receiver 706.
Signals 702 are electromagnetic waves With additional reference to FIG. 5, the interaction of signals 702 induces magnetic fields within the material of stacked core 502. The magnetic fields within stacked core 502 induce a current in electrical excitation component 512. As the electromagnetic fields change within signals 702, the magnitude and/or polarity of the magnetic fields within stacked core 502 similarly change This change in the magnetic fields corresponds to current 704. Receiver 706 is able to receive current 704, and changes therein, to decode signals 702, In this manner, magnetic dipole antenna 602 is a passive device, receiving a signal.
The physical and functional differences between an electric monopole antenna and a magnetic dipole antenna produce different transmission results. These differences will now be described with reference to FIG. 8.
FIG. 8 is a graph 800 illustrating gain as a function of angle for an electric monopole antenna and a stacked antenna.
As shown in the figure, graph 800 includes a y-axis 802 measuring gain in dB, and an x-axis 804 measuring an angle in degrees (from zenith, or along the axis of the electric monopole or magnetic dipole). Graph 800 includes a function 806 and a function 808.
Function 806 corresponds to vertical elevation cut of the radiation performance of a vertical electric monopole antenna 822, as shown to the left of graph 800. Function 806 additionally corresponds to fields discussed above with reference to FIG. 1. Function 806 has a maximum gain at approximately 45° (from zenith) indicated at point 810 and approximately −45° (from zenith) indicated at point 812. The gain drops at 0° (zenith) indicated at point 814, at −180° (nadir) shown at point 816 and at 180° (nadir, not shown), because the radiation is not in the direction along the axis of the electric monopole.
Function 808 corresponds to a vertical elevation cut of the radiation performance of a horizontal magnetic dipole antenna 824, as shown to the right of graph 800 Function 808 additionally corresponds to fields discussed above with reference to FIG. 2. The field distribution and polarization is exactly what one would expect from a magnetic dipole antenna. Function 808 has a maximum gain at approximately 0° (zenith) indicated at point 818. The gain drops at approximately −135° indicated at point 820, and at approximately 135° (not shown), because the horizon is at ±90°, and very little power radiates behind the finite ground plane. Only a small amount of power is diffracted around the ground plane edges, creating the −10 dBi backlobe at nadir.
As shown in FIG. 8 the gain as a function of the angle from azimuth is different for a magnetic dipole antenna as compared to that of an electric monopole antenna. These different gain functions may have different optimal applications. On the other hand, a tall narrow stick-like shape of an electric monopole antenna is quite different from the shape of a stacked-core, bar shape of a magnetic dipole antenna, for example as shown in FIG. 5C. These different shapes may have different optimal applications. There may be situations where the gain function of an electric monopole antenna is desired, but the smaller height of the magnetic dipole antenna is also desired.
What is needed is an antenna that provides a transmission function similar to a conventional electric monopole antenna, but without the large height associated with the conventional electric monopole antenna.